Electronic devices typically include circuits assembled from components that are soldered to conductive traces on a printed circuit board. The board itself is formed of a material such as fiberglass, and has conductive copper traces running along at least one plane of the circuit board. Sophisticated circuit boards often have several layers of traces, including layers on the top and bottom side of the circuit board and embedded within the circuit board. These various traces connect components to one another, and distribute signals such as power, ground, and clock signals throughout the circuit board.
Electrical signals pass through the conductive copper traces between components at high speeds or high frequencies in modern devices, making transmission line effects important to understanding how the signals travel. At high frequencies, the dielectric constant (also known as the relative permittivity) of the material surrounding the conductors affects the speed of propagation of a signal within the conductors. This constant describes the way in which an electric field penetrates a specific material relative to free air. It is also important for understanding how fast a signal travels in a conductor, as propagation delay of a signal in a conductor is proportional to the square root of the dielectric constant. Because the propagation delay of a signal traveling through a conductor in air is approximately 85 picoseconds per inch, we can determine the propagation delay of a signal traveling in the same conductor surrounded by another medium by multiplying 85 picoseconds per inch by the square root of the dielectric constant of the new medium.
The dielectric constant of fiberglass printed circuit boards, such as the common FR4-type circuit board, is approximately 4.5, meaning that a signal propagating on a conductive circuit trace entirely within the FR4 material known as a stripline experiences a delay of the square root of 4.5 multiplied by 85 picoseconds per inch, or approximately 180 picoseconds per inch. Circuit traces on the top and bottom surfaces of the FR4 circuit board are known as microstrips, and because they are surrounded by a combination of free space and FR4 material, experience an effective dielectric constant of about 2.8, resulting in a propagation delay of approximately 140 picoseconds per inch.
A circuit designer can account for these differences in laying out conductive traces on and within a printed circuit board, ensuring that signals take a desired or known time to travel between components. The situation is complicated, however, when various outside forces cause the electric fields surrounding a conductor to vary in configuration, resulting in a changing effective dielectric constant around a microstrip line as a circuit operates.
One example of such a circumstance can occur when two microstrip lines run parallel and near to one another on a surface of a circuit board above a ground plane within the circuit board. When the conductors are carrying signals near one another in voltage or potential (known as even mode), the electric field surrounding each conductor is spatially different in than when the two conductors are carrying signals of different voltages (known as odd mode). When the conductors are at differing potentials, a greater portion of the electric field resides in free space, resulting in a reduction in propagation delay. The change in propagation delay with changed signal mode also results in a significant increase in crosstalk between conductors as observed at the receiving or far end of the conductors, which is also undesirable.
It is therefore desired to reduce the change in propagation delay between even and odd mode signals in microstrip lines.